Adjustable speed ac induction motor drive is known from U.S. Pat. No. 4,137,489 and makes use of a common voltage converter for applying a voltage of variable magnitude to a dc link and a polyphase current source inverter having a variable frequency output with the dc link current magnitude.
As far as the invention is concerned, subsea installations, vessels and vehicles are usually used in exploring and exploiting gas and oil fields at wellheads located at the seabed. Variable Speed Drives are used in this context for applications requiring translatory or rotational movements by electric motors.
Prior art Variable Speed Drives for such subsea installations, vessels and vehicles make use of common voltage-controlled alternating current/alternating current (AC/AC) converters based on insulated gate bipolar transistors (IGBT), the converters comprising a rectifier, a direct current (DC) link buffered by a capacitor bank, and an inverter.
Disadvantageously, capacitors are sensitive to environment pressure. Prior art subsea applications therefore have to provide pressure compensation with an internal air pressure equal to the atmospheric standard pressure of approximately 1013 hPa (1 atm). Typically, the alternating current/alternating current converter and its control electronics are placed in a special housing to keep the pressure-compensated volume small. Only the interior of the housing has to be kept at the atmospheric standard pressure then. As this housing has to be pressure-resistant against the overpressure of the subsea environment or of a surrounding main vessel, several constructional problems occur. Securely withstanding the pressure requires thick walls (up to more than 100 mm depending on pressure and dimensions of the housing) and/or support rods, which takes much space, and poses high prerequisites regarding the sealing of the housing, in particular regarding penetrators for electrical connections into and out of the housing. Additionally, the converter circuits need a complex cooling system to dissipate waste heat from within their housing.